The invention is directed to the field of spent coal utilization as an energy source. In particular, it is concerned with in situ burning of coal where steam is generated as a power source for driving a steam turbine generator, without bringing gaseous contaminates to the surface.
There are several patents in the field of spent coal utilization, the inventor is aware of the following:
U.S. Pat. No. 3,379,248, Apr. 23, 1968; U.S. Pat. No. 4,089,372, May 16, 1978; U.S. Pat. No. 4,089,373, May 16, 1978; U.S. Pat. No. 4,169,506, Oct. 2, 1979.
In U.S. Pat. No. 3,379,248, water is injected into a burning coal formation and steam is removed. Water is forced into the formation through inlet wells and steam is removed through an outlet well. Steam leaving the formation carries hydrocarbons and residual combustion gases to the surface where contaminates may be released.
U.S. Pat. No. 4,089,372 discloses a method for producing heat from coal, among other things. In situ burning of a coal formation generates combustible gases and coal tar mists are driven from the coal for condensation at the surface. The gases and coal tar mists are removed from the formation through a heat exchanger where part of the heat is used to convert water to steam. The steam produced is used to drive a steam turbine generator. The amount of steam produced is dependent upon the heat transfer from the removed gases and mists. This is substantially reduced because of the location of the heat exchange above the burning coal.
A closed loop steam generator is used in U.S. Pat. No. 4,089,373 to drive a steam turbine. The in-situ coal burning process involves positioning a closed loop generator in the area of burning to water converted into steam. The closed loop generator may have different forms, the one form of interest has continuous conduit which consists of a pipe which passes condensed steam from a turbine downward through a first pipe which joins a second vertical pipe. The condensed steam goes down as hot water and is heated and passes upward through the second pipe as steam.
U.S. Pat. No. 4,169,506 is directed towards a process for in-situ retorting of oil shale and energy recovery from generated off gases. A purification zone is used to remove gas impurities that would be detrimental to the environment. The purified gases are used to operate a gas turbine to produce a source of energy.
The combined effect of the above patents fall short of the present invention. A review of these patents will show that the method and apparatus of this invention is beyond the scope of any one of the patented in-situ energy source.